Laser scanning systems are well known in the art of optical inspection. Typically, a laser beam scans over the substrate under inspection, and the light scattered from the substrate is collected by a detector. The intensity (and possibly other characteristics) of the scattered light is compared to an expected range of values. Deviations of the scattered light from the expected range can be indicative of defects in or on the substrate. In integrated circuit (IC) manufacturing, for example, as the laser beam scans the area of a given die on a wafer, the detector signal is compared to a reference signal from another die (“die-to-die” mode) or to stored reference values (“die-to-database” mode). Defects on the wafer are noted wherever the signal deviates from the reference.
The sensitivity of a defect detection system depends on its signal/noise ratio (SNR). In this context, the signal corresponds to the amount of light scattered from the defect that is able to reach the detector. The “noise” is generally dominated by background light that is reflected or otherwise scattered from the substrate itself. (In the context of the present patent application and in the claims, the term “scattered” refers to all radiation returned from the surface, due to substantially any physical mechanism, including diffractive scattering and both specular and diffuse reflection.) When a patterned semiconductor substrate is illuminated with coherent light, for example, the light is diffracted from the repetitive pattern and generates constructive interference lobes along well-defined directions. The positions and extent of the interference lobes depend on the period of the pattern, as well as the wavelength of the incident radiation and characteristics of the optical system.
It is known in the art that blocking the interference lobes can facilitate the detection of defects and pattern irregularities on the substrate. For example, U.S. Pat. No. 3,614,232, to Mathisen, whose disclosure is incorporated herein by reference, describes a spatial filter for detecting defects in photomasks, using a transmission geometry and a simple filter consisting substantially of the negative of the Fourier transform of a defect-free specimen of the microcircuit. There are many cases, however, in which the interference lobes occupy substantially the entire field of view of the detector, so that blocking the lobes is impossible without entirely blocking the detector. Even when a portion of the field of view of the detector is still available between the interference lobes, this portion may be so limited that too small a signal remains after blocking the regions of the lobes.